Process for the manufacture of a composition comprising ethylene polymers

ABSTRACT

Process for the manufacture of a composition comprising ethylene polymers, in at least two polymerization reactors connected in series, according to which, in a first reactor, from 30 to 70% by weight with respect to the total weight of the composition of an ethylene homopolymer (A) having a melt flow index MI 2  of 5 to 1000 g/10 min is formed and, in a subsequent reactor, from 30 to 70% by weight with respect to the total weight of the composition of a copolymer of ethylene and of hexene (B) having a melt flow index MI 5  of 0.01 to 2 g/10 min is formed. The compositions obtained by this process exhibit a good compromise between the processing properties and the mechanical properties, which renders them capable of being used in the manufacture of articles shaped by extrusion and extrusion blow-molding, such as films and pipes.

[0001] The present invention relates to a process for the manufacture ofa composition comprising ethylene polymers, comprising a homopolymer anda copolymer of ethylene, which makes use of several polymerizationreactors connected in series. It also relates to the compositionscomprising ethylene polymers capable of being obtained by this processand to their use in the manufacture of films and pipes.

[0002] Patent Application EP-A-0,603,935 (Solvay) discloses a processfor the preparation of a composition comprising ethylene polymerscomprising an ethylene polymer with a high melt flow index (MI₂ of 5 to1000 g/10 min) and an ethylene polymer with a low melt flow index (MI₅of 0.01 to 2 g/10 min) in at least two reactors arranged in series, theratio by weight of these polymers being equal to (30 to 70):(70 to 30).This patent application more specifically discloses a composition,prepared in suspension in hexane, comprising an ethylene homopolymerhaving an MI₂ of 168 g/10 min and a copolymer of ethylene and of butenehaving an MI₅ of 0.21 g/10 min.

[0003] Patent Application EP-A-0,580,930 discloses a process for thepreparation of a composition comprising ethylene polymers in two loopreactors in the liquid phase in which, in a first reactor, ethylene andan alpha-olefin, such as hexene, are introduced, so as to prepare acopolymer of ethylene and of hexene having a melt flow index HLMIvarying from 0.01 to 5 g/10 min, and then the mixture resulting from thefirst reactor is introduced into a second reactor fed with ethylene, soas to obtain a polymer of ethylene having an HLMI of greater than 5 g/10min. As the mixture resulting from the first reactor still comprisesunpolymerized hexene, the polymer formed in the second reactor is also acopolymer of ethylene and of hexene.

[0004] The compositions resulting from these processes generally exhibitprocessing and mechanical properties which render them suitable forbeing used in the manufacture of various shaped articles.

[0005] The aim of the present invention is to provide a process for themanufacture of compositions comprising ethylene polymers exhibiting abetter compromise between the processing properties and the mechanicalproperties in comparison with the compositions obtained by the knownprocesses of the state of the art.

[0006] The invention consequently relates to a process for themanufacture of a composition comprising ethylene polymers, in at leasttwo polymerization reactors connected in series, according to which:

[0007] in a first reactor, ethylene is polymerized in suspension in amixture comprising a diluent, hydrogen, a catalyst based on a transitionmetal and a cocatalyst, so as to form from 30 to 70% by weight withrespect to the total weight of the composition of an ethylenehomopolymer (A) having a melt flow index MI₂ of 5 to 1000 g/10 min,

[0008] the said mixture, additionally comprising the homopolymer (A), iswithdrawn from the said reactor and is subjected to a reduction inpressure, so as to degas at least a portion of the hydrogen, then

[0009] the said at least partially degassed mixture comprising thehomopolymer (A), as well as ethylene and 1-hexene and, optionally, atleast one other α-olefin, are introduced into a subsequent reactor andthe suspension polymerization is carried out therein in order to formfrom 30 to 70% by weight, with respect to the total weight of thecomposition, of a copolymer of ethylene and of hexene (B) having a meltflow index MI₅ of 0.01 to 2 g/10 min.

[0010] For the purposes of the present invention, ethylene homopolymer(A) is understood to denote an ethylene polymer composed essentially ofmonomer units derived from ethylene and substantially devoid of monomerunits derived from other olefins. Copolymer of ethylene and of hexene(3) is understood to denote a copolymer comprising monomer units derivedfrom ethylene and monomer units derived from 1-hexene and, optionally,from at least one other α-olefin. The other α-olefin can be selectedfrom olefinically unsaturated monomers comprising from 3 to 8 carbonatoms (with the exclusion of 1-hexene), such as, for example, propylene,1-butene, 1-pentene, 3-methyl-1-butene, 3- and 4-methyl-1-pentenes and1-octene. Preferred α-olefins are propylene, 1-butene and 1-octene andmore particularly still 1-butene. The copolymer (B) according to theinvention generally comprises at least 90%, in particular at least 94%,by weight of monomer units derived from ethylene. It preferablycomprises at least 96% by weight of monomer units derived from ethylene.The content of monomer units derived from 1-hexene in the copolymer (B),hereinafter referred to as hexene content, is generally at least 0.4% byweight, in particular at least 0.6% by weight, values of at least 1% byweight being favourable. The hexene content of the copolymer (B) isusually at most 10% by weight, preferably at most 6% by weight. A hexenecontent which does not exceed 4% by weight is particularly preferred.For the purposes of the present invention, the hexene content of thecopolymer (B) is measured by ¹³C NMR according to the method describedin J. C. Randall, JMS-Rev. Macromol. Chem. Phys., C29(2&3), p. 201-317(1989), that is to say that the content of units derived from hexene iscalculated from the measurements of the integrals of the linescharacteristic of hexene (23.4, 34.9 and 38.1 ppm), in comparison withthe integral of the line characteristic of the units derived fromethylene (30 ppm). A copolymer (B) composed essentially of monomer unitsderived from ethylene and from 1-hexene is particularly preferred.

[0011] For the purposes of the present invention, melt flow index MI₂,respectively MI₅, is understood to denote the melt flow indices measuredaccording to ASTM Standard D 1238 (1986) at a temperature of 190° C.under a load of 2.16 kg, respectively 5 kg. Furthermore, melt flow indexHLMI is understood to denote the melt flow index measured according toASTM Standard D 1238 (1986) at a temperature of 190° C. under a load of21.6 kg.

[0012] The homopolymer (A) according to the invention preferablyexhibits an MI₂ of at least 50, very particularly of at least 90, g/10min. The MI₂ of the homopolymer (A) preferably does not exceed 700 g/10min. The homopolymer (A) advantageously exhibits an HLMI of at least 100g/10 min.

[0013] The homopolymer (A) advantageously exhibits an intrinsicviscosity η_(A) (measured in tetrahydronaphthalene at 160° C.) of atleast 0.50 dl/g, preferably of at least 0.58 dl/g. Its intrinsicviscosity generally does not exceed 1.50 dl/g, preferably it does notexceed 1.00 dl/g. A homopolymer for which η_(A) does not exceed 0.86dl/g is particularly preferred.

[0014] The melt flow index MI₅ of the copolymer (B) according to theinvention is preferably at least 0.015 g/10 min. It preferably does notexceed 0.1 g/10 min. The copolymer (B) advantageously exhibits an HLMIof at least 0.1 g/10 min which, furthermore, does not exceed 20 g/10min.

[0015] The copolymer (B) generally exhibits an intrinsic viscosity η_(B)(measured in tetrahydronaphthalene at 160° C.) of at least 2.20 dl/g.Its intrinsic viscosity η_(B) generally does not exceed 6.30 dl/g,preferably not 5.90 dl/g. A copolymer (B) for which the intrinsicviscosity does not exceed 4.00 dl/g is particularly preferred.

[0016] Suspension polymerization is understood to denote thepolymerization in a diluent which is in the liquid state under thepolymerization conditions (temperature, pressure) used, thesepolymerization conditions or the diluent being such that at least 50% byweight (preferably at least 70%) of the polymer formed is insoluble inthe said diluent.

[0017] The diluent used in the polymerization process according to theinvention is usually a hydrocarbon-comprising diluent which is inertwith respect to the catalyst, the cocatalyst and the polymer formed,such as, for example, a linear or branched alkane or a cycloalkanehaving from 3 to 8 carbon atoms. The diluent which has given the bestresults is isobutane. One advantage of the use of isobutane lies inparticular in its ready recycling. This is because the use of isobutanemakes it possible to recycle the diluent recovered at the end of theprocess according to the invention in the first reactor without havingto carry out exhaustive purification in order to remove the residualhexene. This is because, as the boiling temperatures of isobutane and ofhexene are far apart, their separation can be carried out bydistillation.

[0018] The amount of ethylene introduced into the first polymerizationreactor and into the subsequent polymerization reactor is generallyadjusted so as to obtain a concentration of ethylene in the diluent of 5to 50 g of ethylene per kg of diluent.

[0019] The amount of hydrogen introduced into the first reactor isgenerally adjusted so as to obtain, in the diluent, a molar ratio ofhydrogen to ethylene of 0.05 to 1. In the first reactor, this molarratio is preferably at least 0.1. A hydrogen/ethylene molar ratio whichdoes not exceed 0.6 is particularly preferred.

[0020] The mixture withdrawn from the first reactor, additionallycomprising the homopolymer (A), is subjected to a reduction in pressureso as to remove (degas) at least a portion of the hydrogen. Thereduction in pressure is advantageously carried out at a temperature ofless than or equal to the polymerization temperature in the firstreactor. The temperature at which the reduction in pressure is carriedout is usually greater than 20° C.; it is preferably at least 40° C. Thepressure at which the reduction in pressure is carried out is less thanthe pressure in the first reactor. The pressure-reduction pressure ispreferably less than 1.5 MPa. The pressure-reduction pressure is usuallyat least 0.1 MPa. The amount of hydrogen still present in the at leastpartially degassed mixture is generally less than 1% by weight of theamount of hydrogen initially present in the mixture withdrawn from thefirst polymerization reactor; this amount is preferably less than 0.5%.The amount of hydrogen present in the partially degassed mixtureintroduced into the subsequent polymerization reactor is consequentlylow, or even zero. The subsequent reactor is preferably also fed withhydrogen. The amount of hydrogen introduced into the subsequent reactoris generally adjusted so as to obtain, in the diluent, a molar ratio ofhydrogen to ethylene of 0.001 to 0.1. This molar ratio is preferably atleast 0.004 in this subsequent reactor. It preferably does not exceed0.05. In the process according to the invention, the ratio of theconcentration of hydrogen in the diluent in the first reactor to theconcentration in the subsequent polymerization reactor is usually atleast 20, preferably at least 30. A ratio of concentrations of at least40 is particularly preferred. This ratio usually does not exceed 300,preferably not 200.

[0021] The amount of 1-hexene introduced into the subsequentpolymerization reactor is such that, in this reactor, thehexene/ethylene molar ratio in the diluent is at least 0.05, preferablyat least 0.1. The amount of hexene introduced into the subsequentreactor is such that the hexene/ethylene molar ratio does not exceed 3,preferably not 2.8. In the process according to the invention, the firstreactor is not fed with hexene. It is essential that the first reactoris essentially devoid of 1-hexene. Consequently, the diluent introducedinto the first reactor, which can be recycled diluent, must be highlydepleted in hexene. The diluent introduced into the first reactorpreferably contains less than 1000 ppm of hexene. In a particularlypreferred way, the diluent introduced into the first polymerizationreactor is essentially devoid of hexene.

[0022] The catalyst used in the process according to the inventioncomprises at least one transition metal. Transition metal is understoodto denote a metal from Groups 4, 5 or 6 of the Periodic Table of theElements (CRC Handbook of Chemistry and Physics, 75th edition, 1994-95).The transition metal is preferably titanium and/or zirconium. Titaniumis particularly preferred. In the process according to the invention,use is preferably made of a catalyst comprising, in addition to thetransition metal, magnesium. Good results have been obtained withcatalysts comprising:

[0023] from 10 to 30%, preferably from 15 to 20%, by weight oftransition metal,

[0024] from 0.5 to 20%, preferably from 1 to 10%, by weight ofmagnesium,

[0025] from 20 to 60%, preferably from 30 to 50%, by weight of halogen,such as chlorine,

[0026] from 0.1 to 10%, preferably from 0.5 to 5%, by weight ofaluminium;

[0027] the balance generally being composed of elements originating fromthe products used in their manufacture, such as carbon, hydrogen andoxygen. These catalysts are preferably obtained by coprecipitation of atleast one transition metal compound and of a magnesium compound by meansof a halogenated organoaluminium compound. Such catalysts are known;they have been disclosed particularly in patents U.S. Pat. No.3,901,863, U.S. Pat. No. 4,929,200 and U.S. Pat. No. 4,617,360 (Solvay).In the process according to the invention, the catalyst is preferablyintroduced solely into the first polymerization reactor, that is to saythat fresh catalyst is not introduced into the subsequent polymerizationreactor. The amount of catalyst introduced into the first reactor isgenerally adjusted so as to obtain an amount of at least 0.5 mg oftransition metal per liter of diluent. The amount of catalyst usuallydoes not exceed 100 mg of transition metal per liter of diluent.

[0028] The cocatalyst employed in the process according to the inventionis preferably an organoaluminium compound. Non-halogenatedorganoaluminium compounds of formula AlR₃ in which R represents an alkylgroup having from 1 to 8 carbon atoms are preferred. Triethylaluminiumand triisobutylaluminium are particularly preferred. The cocatalyst isintroduced into the first polymerization reactor. It is also possible tointroduce fresh cocatalyst into the subsequent reactor. The amount ofcocatalyst introduced into the first reactor is generally at least0.1×10⁻³ mol per liter of diluent. It usually does not exceed 5×10⁻³ molper liter of diluent. If appropriate, the amount of fresh cocatalystintroduced into the subsequent reactor usually does not exceed 5×10⁻³mol per liter of diluent.

[0029] The polymerization temperature is generally from 20 to 130° C. Itis preferably at least 60° C. It preferably does not exceed 115° C. Thetotal pressure at which the process according to the invention iscarried out is generally from 0.1 MPa to 10 MPa. In the firstpolymerization reactor, the total pressure is preferably at least 2.5MPa. It preferably does not exceed 5 MPa. In the subsequentpolymerization reactor, the total pressure is preferably at least 1.3MPa. It preferably does not exceed 4.3 MPa.

[0030] The duration of polymerization in the first reactor and in thesubsequent reactor is generally at least 20 minutes, preferably at least30 minutes. The duration of polymerization usually does not exceed 5hours, preferably not 3 hours.

[0031] In order to carry out the process according to the invention, itis possible to make use of a plant comprising more than twopolymerization reactors connected in series. It is preferable torestrict the system to two polymerization reactors connected in series,separated by a device which makes it possible to carry out the reductionin pressure.

[0032] In the process according to the invention, a suspensioncomprising a composition comprising from 30 to 70% by weight of thehomopolymer (A) and from 30 to 70% by weight of the copolymer (B) iscollected at the outlet of the subsequent polymerization reactor. Thecomposition comprising ethylene polymers can be separated from thesuspension by any known means. The suspension is usually subjected to areduction in pressure (final reduction in pressure), so as to remove thediluent, the ethylene, the hexene and, optionally, the hydrogen from thecomposition. According to an alternative form of the process accordingto the invention and more particularly when the diluent is isobutane,the gases exiting from the first reduction in pressure (intermediatereduction in pressure between the two polymerization reactors) and fromthe final reduction in pressure are mixed, compressed and conveyed to adistillation unit. This distillation unit is advantageously composed ofone or of two distillation columns in series. Ethylene and hydrogen arewithdrawn at the column top, a mixture of isobutane and of hexene iswithdrawn at the column bottom and isobutane devoid of hexene iswithdrawn from an intermediate plate. The isobutane-hexene mixture isthen recycled in the subsequent polymerization reactor, whereas theisobutane devoid of hexene is recycled in the first reactor.

[0033] The process according to the invention makes it possible toobtain, with a good yield and with a low content of oligomers, acomposition comprising ethylene polymers exhibiting a very goodcompromise between the mechanical properties and the processingproperties.

[0034] The invention consequently also relates to a compositioncomprising ethylene polymers comprising from 30 to 70% by weight withrespect to the total weight of the composition of an ethylenehomopolymer (A) having a melt flow index MI₂ of 5 to 1000 g/10 min andfrom 30 to 70% by weight with respect to the total weight of thecomposition of a copolymer of ethylene and of hexene (B) having a meltflow index MI₅ of 0.01 to 2 g/10 min capable of being obtained by theprocess according to the invention.

[0035] An essential characteristic of the composition according to theinvention is that it is composed of an intimate and homogeneous mixtureof the homopolymer (A) and of the copolymer (B), the copolymer (B) beingprepared in the presence of the homopolymer (A). The composition iscomposed of particles comprising both homopolymer (A) and copolymer (B).

[0036] The amount of homopolymer (A) in the composition comprisingethylene polymers according to the invention is preferably at least 40%,more particularly at least 42%, by weight with respect to the totalweight of the composition. The amount of homopolymer (A) preferably doesnot exceed 60% by weight. Good results have been obtained with an amountof homopolymer (A) which does not exceed 58% by weight with respect tothe total weight of the composition.

[0037] The amount of copolymer (B) is preferably at least 40%, moreparticularly at least 42%, by weight with respect to the total weight ofthe composition. The amount of copolymer (B) preferably does not exceed60% by weight. Good results have been obtained with an amount ofcopolymer (B) not exceeding 58% by weight with respect to the totalweight of the composition.

[0038] The composition according to the invention generally comprises atleast 95%, preferably at least 99%, by weight of the combination of thehomopolymer (A) and of the copolymer (B). A composition composed solelyof the homopolymer (A) and of the copolymer (B) is very particularlypreferred.

[0039] The composition according to the invention generally exhibits amelt flow index MI₅ of at least 0.07 g/10 min, preferably of at least0.1 g/10 min. The MI₅ of the composition usually does not exceed 10 g/10min, preferably not 7 g/10 min. Compositions for which the MI₅ does notexceed 1 g/10 min are particularly preferred. The composition accordingto the invention advantageously exhibits an HLMI of at least 2 g/10 minwhich, furthermore, does not exceed 100 g/10 min.

[0040] An important characteristic of the composition according to theinvention is that it exhibits a broad or bimodal molecular weightdistribution. This characteristic is illustrated by the ratio of themelt flow indices measured under various loads and more specifically bythe HLMI/MI₅ ratio. The compositions usually exhibit an HLMI/MI₅ ratioof greater than 10, preferably greater than 15. The HLMI/MI₅ ratiousually does not exceed 150. The HLMI/MI₅ ratio preferably does notexceed 50. In the compositions according to the invention, the ratio ofthe intrinsic viscosity of the copolymer (B)(η_(B)) to that of thehomopolymer (A)(η_(A)) is generally at least 1.5, preferably at least 2.The η_(B)/η_(A) ratio generally does not exceed 12, preferably not 10. Aratio which does not exceed 7 is particularly preferred.

[0041] In addition, the composition according to the invention usuallyexhibits a dynamic viscosity μ₂, measured at 190° C. at a rate gradientof 100 s⁻¹, of 10 to 30,000 dpa·s. In the context of the presentinvention, the dynamic viscosity μ₂ is determined by extrusion of thepolymer at 190° C., through a die with a length of 15 mm and a diameterof 1 mm, at a constant rate corresponding to a rate gradient of 100 s⁻¹and by measuring the force transmitted by the piston during its descent.The dynamic viscosity μ₂ is then calculated from the relationshipμ₂=233×Fp, in which Fp represents the mean force exerted by the piston,expressed in daN, during the measuring time of 30 seconds. The cylinderand the piston of the rheometer which are used for this measurementcorrespond to the criteria of that used for the measurement of the meltflow index according to ASTM Standard D 1238 (1986).

[0042] The compositions according to the invention generally exhibit astandard density SD, measured according to ASTM Standard D 792 (on asample prepared according to ASTM Standard D 1928, Procedure C), of atleast 930 kg/m³. The compositions preferably exhibit an SD of greaterthan 935 kg/m³. Compositions which have given good results are those forwhich the SD is at least equal to 940 kg/m³. The SD generally does notexceed 965 kg/m³; it preferably does not exceed 960 kg/m³. Compositionsfor which the SD is less than 955 kg/m³ are particularly preferred. TheSD of the homopolymer (A) present in the compositions according to theinvention is generally at least 960 kg/m³, preferably at least 965kg/m³. A homopolymer (A) having an SD of at least 970 kg/m³ is veryparticularly preferred. The SD of the copolymer (B) is generally from910 to 940 kg/m³. The SD of the copolymer (B) is preferably at least 915kg/m³. The SD of the copolymer (B) preferably does not exceed 938 kg/m³,more particularly not 935 kg/m³.

[0043] The compositions according to the invention are suitable forbeing employed according to conventional processes for shaping articlesand more particularly according to extrusion and extrusion blow-mouldingprocesses.

[0044] The compositions according to the invention are well suited tothe manufacture of films. The invention consequently also relates to theuse of a composition according to the invention for the manufacture offilms, in particular by extrusion blow-moulding, and to the filmsproduced by use of the composition according to the invention. Thecompositions according to the invention make it possible to obtain filmsexhibiting both a beautiful surface appearance (absence of defects knownas shark skin) and a good resistance to tearing and to perforation.

[0045] The compositions according to the invention are particularly wellsuited to the extrusion of pipes, in particular pipes for thetransportation of pressurized fluids, such as water and gas. Theinvention consequently also relates to the use of a compositionaccording to the invention for the manufacture of pipes. The manufactureof pipes by extrusion of a composition according to the invention isadvantageously carried out on an extrusion line comprising an extruder,a sizer and a haul-off device. The extrusion is generally carried out onan extruder of the single-screw type and at a temperature of 150 to 230°C. The sizing of the pipes can be carried out by the creation of anegative pressure within the pipe and/or by the creation of an excesspressure outside the pipe.

[0046] The pipes manufactured by means of the compositions according tothe invention are characterized by:

[0047] a good resistance to slow crack propagation or environmentalstress cracking resistance (ESCR), reflected by a failure time generallyof greater than 2000 hours, as measured at 80° C. on a notched pipehaving a diameter of 110 mm and a thickness of 10 mm and under a stressof 4.6 MPa according to the method described in ISO Standard FIDIS 13479(1996),

[0048] a good resistance to rapid crack propagation (RCP), reflected bya halt in crack propagation at an internal pressure generally at leastequal to 12 bar, as measured at 0° C. on a pipe with a diameter of 110mm and a thickness of 10 mm according to the S4 method described in ISOStandard F/DIS 13477 (1996), and

[0049] a good creep resistance (τ), reflected by a failure timegenerally of greater than 200 hours (measured at 20° C. on a pipe havinga diameter of 50 mm and a thickness of 3 mm under a circumferentialstress of 12.4 MPa according to ISO Standard 1167).

[0050] The pipes manufactured by means of the compositions according tothe invention are characterized in particular by a better compromisebetween the resistance to crack propagation (slow crack propagation andrapid crack propagation) and the creep resistance in comparison with theknown compositions of the prior art. The invention consequently alsorelates to the pipes, more particularly the pipes for the transportationof pressurized fluids, obtained by extrusion of a composition accordingto the invention.

[0051] It goes without saying that, when they are used for the moltenshaping of articles, the compositions according to the invention can bemixed with the usual processing additives for polyolefins, such asstabilizers (antioxidizing agents and/or anti-UV agents), antistaticagents and processing aids, as well as pigments. The inventionconsequently also relates to a mixture comprising a compositionaccording to the invention and at least one of the additives describedabove. The mixtures comprising at least 95%, preferably at least 97%, byweight of a composition according to the invention and at least one ofthe additives described above are particularly preferred.

[0052] The examples which follow are intended to illustrate theinvention.

[0053] The meanings of the symbols used in these examples and the unitsexpressing the properties mentioned and the methods for measuring theseproperties are explained below.

[0054] Q=content of comonomer in the copolymer (B), expressed as % byweight. In the case of hexene, the content was measured as describedabove; in the case of butene, the butene content was also measured byNMR according to the method described above, but by using the linescharacteristic of butene (11.18 and 39.6 ppm).

[0055] QT=content of comonomer in the composition, expressed as % byweight. This amount is measured as explained above for the comonomercontent of the copolymer (B).

[0056] Elmendorf=resistance to tearing measured according to ASTMStandard D 1922-67; L denotes the measurement in the longitudinaldirection of the film, T denotes the measurement in the transversedirection of the film.

[0057] DDT=resistance to perforation measured according to ISO Standard7765-1 (Dart Drop Test). The values have been expressed in g perthickness of the film in μm.

[0058] The other symbols have been explained in the description.

[0059] The values labelled * have been calculated from the valuesmeasured for the polymer manufactured in the reactor 1 and for thecomposition resulting from the reactor 2.

EXAMPLES 1, 2, 4 AND 7

[0060] a) Preparation of the catalyst

[0061] Magnesium diethoxide was reacted with titanium tetrabutoxide for4 hours at 150° C. in an amount such that the molar ratio of titanium tomagnesium is equal to 2. The reaction product thus obtained wassubsequently chlorinated and precipitated by bringing the latter intocontact with an ethylaluminium dichloride solution for 90 minutes at 45°C. The catalyst thus obtained, collected from the suspension, comprises(% by weight):

[0062] Ti: 17; Cl: 41; Al: 2; Mg: 5.

[0063] b) Preparation of the composition

[0064] The manufacture of a composition comprising ethylene polymers wascarried out in suspension in isobutane in two loop reactors connected inseries separated by a device which makes it possible continuously tocarry out the reduction in pressure.

[0065] Isobutane, ethylene, hydrogen, triethylaluminium and the catalystdescribed in point a) were continuously introduced into the first loopreactor and the polymerization of ethylene was carried out in thismixture in order to form the homopolymer (A). The said mixture,additionally comprising the homopolymer (A), was continuously withdrawnfrom the said reactor and was subjected to a reduction in pressure (60°C., 0.7 MPa), so as to remove at least a portion of the hydrogen. Theresulting mixture, at least partially degassed of hydrogen, was thencontinuously introduced into a second polymerization reactor, at thesame time as ethylene, hexene, isobutane and hydrogen, and thepolymerization of the ethylene and of the hexene was carried out thereinin order to form the copolymer (B). The suspension comprising thecomposition comprising ethylene polymers was continuously withdrawn fromthe second reactor and this suspension was subjected to a finalreduction in pressure, so as to evaporate the isobutane and thereactants present (ethylene, hexene and hydrogen) and to recover thecomposition in the form of a powder, which was subjected to drying inorder to complete the degassing of the isobutane.

[0066] The other polymerization conditions are specified in Table 1.

[0067] The properties of the final compositions are presented in Table2.

[0068] c) Use of the composition for the preparation of films.

[0069] The compositions of the various examples were used for themanufacture of films by extrusion blow-moulding through a die with adiameter of 100 mm, with a blow ratio (ratio of the diameter of thebubble to the diameter of the extrusion die) set at 4 and a neck heightof 6 times the diameter of the extrusion die. The mechanical propertiesof the films obtained are presented in Table 2.

COMPARATIVE EXAMPLE 3R

[0070] A composition comprising ethylene polymers was manufactured inthe plant and with the catalyst and the cocatalyst described in Example1 but by using hexane as diluent and butene as comonomer in the secondreactor. The other conditions are specified in Table 1.

[0071] The properties of the composition obtained are presented in Table2.

[0072] Films were manufactured with this composition not in accordancewith the invention under the same conditions as for Examples 1, 2, 4 and7. The mechanical properties of the films obtained are also presented inTable 2.

COMPARATIVE EXAMPLES 5R AND 6R

[0073] A composition comprising ethylene polymers was manufactured inthe plant and with the catalyst and the cocatalyst described in Example1 but by using hexene as comonomer in the two polymerization reactors.The other conditions are specified in Table 1.

[0074] The properties of the compositions obtained are presented inTable 2.

[0075] Films were manufactured with these compositions not in accordancewith the invention under the same conditions as for Examples 1, 2, 4 and7. The mechanical properties of the films obtained are also presented inTable 2. TABLE 1 EXAMPLE 1 2 3R 4 5R 6R 7 REACTOR 1 diluent isobutaneisobutane hexane isobutane isobutane isobutane isobutane C₂ (g/kg) 9 914 10 9 8.8 10 comonomer — — — — 1-hexene 1-hexene — comon./C₂ (mol/mol)0 0 0 0 0.37 0.37 0 H₂/C₂ (mol/mol) 0.449 0.447 0.437 0.398 0.370 0.4280.451 T (° C.) 85 85 85 85 85 85 85 residence time (h) 2.20 2.20 2.202.20 2.20 2.20 2.20 REACTOR 2 diluent isobutane isobutane hexaneisobutane isobutane isobutane isobutane C₂ (g/kg) 32 33 13 35 35 38 38comonomer 1-hexene 1-hexene 1-butene 1-hexene 1-hexene 1-hexene 1-hexenecomon./C₂ (mol/mol) 1.52 1.41 0.82 2.72 1.76 1.27 1.64 H₂/C₂ (mol/mol)0.010 0.011 0.009 0.016 0.017 0.021 0.009 T (° C.) 85 85 85 85 85 83 85residence time (h) 1.23 1.23 1.23 1.27 1.27 1.23 1.23

[0076] TABLE 2 EXAMPLE 1 2 3R 4 5R 6R 7 polym. manu- homopolymerhomopolymer homopolymer homopolymer C₂-C₆ C₂-C₆ homopolymer facturedreact. 1 copo copo weight (%) 45 45 45 55 55 45 45 MI₂ (g/10 min) 99.897.6 101.5 116 121 133 102 η_(A) (dl/g) 0.84 0.84 0.84 0.81 0.81 0.790.84 SD (kg/m³) 968 969 969 968 962 961 967 polym. manu- C₂-C₆ C₂-C₆C₂-C₄ C₂-C₆ C₂-C₆ C₂-C₆ C₂-C₆ factured react. 2 copo copo copo copo copocopo copo weight (%) 55 55 55 45 45 55 55 MI₅ * (g/10 min) 0.04 0.050.05 0.015 0.015 0.04 0.04 SD * (kg/m³) 937.5 935 936.8 926.8 935.7939.7 936.5 Q * (weight %) 2.84 2.62 2.33 5.07 3.28 2.37 3.05composition resulting react. 2 QT (weight %) 1.56 1.44 1.28 2.28 1.861.62 1.68 MI₅ (g/10 min) 0.31 0.35 0.37 0.23 0.22 0.28 0.30 HLMI (g/10min) 5.7 6.1 7.4 6.6 6.7 5.7 5.7 SD (kg/m³) 951 950 951 949 950 949 950μ₂ (dPa · s) 27,200 27,500 25,200 22,600 22,300 26,200 26,600 Filmproperties film thick. (μm) 29 30 31 32 32 32 28 DDT (g/μm) 8.4 7.6 6.710.8 9.5 8.5 10.0 Elmendorf L (g) 26 26 23 42 26 Elmendorf T (g) 355 305312 379 288

[0077] Table 2 shows that the compositions comprising a homopolymer andan ethylene/hexene copolymer which are obtained by the process accordingto the invention exhibit better mechanical properties (resistance totearing and resistance to perforation) in comparison with a compositioncomprising butene instead of hexene (Ex. 1 and 2 in comparison with Ex.3R) and in comparison with compositions comprising two hexene copolymers(Ex. 4 in comparison with Ex. 5R and Ex. 7 in comparison with Ex. 6R).

EXAMPLES 8 AND 9R

[0078] These examples were carried out in the plant and with thecatalyst and the cocatalyst described in Example 1. The polymerizationconditions in the two reactors are summarized in Table 3. TABLE 3EXAMPLE 8 9R diluent Isobutane Hexane REACTOR 1 C₂ (g/kg) 14.9 10.7H₂/C₂ 0.46 0.38 T (° C.) 85 85 residence time (h) 2.3 3.1 REACTOR 2 C₂(g/kg) 21.9 14.9 comonomer Hexene Butene comonom./C₂ 1.61 0.89 H₂/C₂0.0048 0.0033 T (° C.) 75 75 residence time (h) 1.3 1.93

[0079] The properties of the final compositions are summarized in Table4.

[0080] 997 parts of the composition obtained were mixed with 2 parts ofan antioxidizing agent and 1 part of an anti-UV agent and the mixturewas granulated by extrusion in an extruder at a temperature of 230° C.

[0081] Pipes were then manufactured by extrusion of these granules on anextruder of single-screw type at 200° C. The properties measured onthese pipes are taken up in Table 4. It is evident that the compositioncomprising an ethylene-hexene copolymer (Ex. 8) exhibits a bettercompromise between the resistance to crack propagation (resistance toslow crack propagation and resistance to rapid crack propagation) andthe creep resistance in comparison with a composition comprising anethylene-butene copolymer (Ex. 9R). TABLE 4 EXAMPLE 8 9R Polymermanufactured in reactor 1 Homo homo weight (%) 50.2 50.8 MI₂ (g/10 min)575 468 η_(A) (dl/g) 0.59 0.62 SD (kg/m³) 973 972 Polymer manufacturedin reactor 2 C₂-C₆ copo C₂-C₄ copo weight (%) 49.8 49.2 MI₅ * (g/10 min)0.03 0.025 SD * (kg/m³) 927.1 925.8 Q * (weight %) 3 2.5 Compositionresulting from react. 2 QT (weight %) 1.5 1.24 MI₅ (g/10 min) 0.31 0.31SD (kg/m³) 949.6 948.7 μ₂ (dPa · s) 22,100 20,500 ESCR (h) >7224 7344RCP (bar) at 0° C. >12 >12 at −15° C. >12 >12 τ (h) 1780 235

EXAMPLE 10

[0082] This example was carried out in the plant of Example 1, with acatalyst comprising, as % by weight, Ti: 5; Zr: 18; Cl: 45; Al: 5; Mg: 6and triisobutylaluminium as cocatalyst. The polymerization conditions inthe two reactors are summarized in Table 5. TABLE 5 EXAMPLE 10 DiluentIsobutane REACTOR 1 C₂ (g/kg) 25.2 H₂/C₂ 0.47 T (° C.) 85 residence time(h) 3.29 REACTOR 2 C₂ (g/kg) 29.8 Comonomer hexene comonom./C₂ 1.32H₂/C₂ 0.0048 T (° C.) 75 residence time (h) 1.86

[0083] The properties of the final compositions are summarized in Table6.

[0084] 997 parts of the composition obtained were mixed with 2 parts ofan antioxidizing agent and 1 part of an anti-UV agent and the mixturewas granulated by extrusion in an extruder at a temperature of 230° C.

[0085] Pipes were then manufactured by extrusion of these granules on anextruder of single-screw type at 200° C. The properties measured onthese pipes are taken up in Table 6. TABLE 6 EXAMPLE 10 Polymermanufactured in reactor 1 homo weight (%) 53.4 MI₂ (g/10 min) 400 SD(kg/m³) 971.8 Polymer manufactured in reactor 2 C₂-C₆ copo weight (%)46.6 MI₅ * (g/10 min) 0.04 SD * (kg/m³) 923.5 Q * (weight %) 3Composition resulting from react. 2 QT (weight %) 1.4 MI₅ (g/10 min)0.54 SD (kg/m³) 948.7 μ₂ (dPa · s) 19,000 ESCR (h) >3600

[0086] Belgin Patent Application 09700694 is hereby incorporated hereinby reference.

1. Process for the manufacture of a composition comprising ethylenepolymers, in at least two polymerization reactors connected in series,according to which: in a first reactor, ethylene is polymerized insuspension in a mixture comprising a diluent, hydrogen, a catalyst basedon a transition metal and a cocatalyst, so as to form from 30 to 70% byweight with respect to the total weight of the composition of anethylene homopolymer (A) having a melt flow index MI₂ of 5 to 1000 g/10min, the said mixture, additionally comprising the homopolymer (A), iswithdrawn from the said reactor and is subjected to a reduction inpressure, so as to degas at least a portion of the hydrogen, then thesaid at least partially degassed mixture comprising the homopolymer (A),as well as ethylene and 1-hexene and, optionally, at least one otherα-olefin are introduced into a subsequent reactor and the suspensionpolymerization is carried out therein in order to form from 30 to 70% byweight, with respect to the total weight of the composition, of acopolymer of ethylene and of hexene (B) having a melt flow index MI₅ of0.01 to 2 g/10 min.
 2. Process according to claim 1 , characterized inthat the hexene content in the copolymer (B) is at least 0.4% and atmost 10% by weight.
 3. Process according to claim 1 , characterized inthat the copolymer (B) is composed essentially of monomer units derivedfrom ethylene and from 1-hexene.
 4. Process according to claim 1 ,characterized in that the homopolymer (A) exhibits an MI₂ of at least 50and not exceeding 700 g/10 min and that the copolymer (B) exhibits anMI₅ of at least 0.015 and not exceeding 0.1 g/10 min.
 5. Processaccording to claim 1 , characterized in that the diluent is isobutane.6. Process according to claim 1 , characterized in that the amount ofhydrogen introduced into the first reactor is adjusted so as to obtain,in the diluent, a molar ratio of hydrogen to ethylene of 0.05 to
 1. 7.Process according to claim 1 , characterized in that the ratio of theconcentration of hydrogen in the first reactor to the concentration inthe subsequent polymerization reactor is at least
 20. 8. Processaccording to claim 1 , characterized in that the amount of 1-hexeneintroduced into the subsequent polymerization reactor is such that, inthis reactor, the hexene/ethylene molar ratio in the diluent is from0.05 to
 3. 9. Process according to claim 1 , characterized in that thecatalyst comprises from 10 to 30% by weight of transition metal, from0.5 to 20% by weight of magnesium, from 20 to 60% by weight of a halogenand from 0.1 to 10% by weight of aluminium.
 10. Composition comprisingethylene polymers comprising from 30 to 70% by weight with respect tothe total weight of the composition of an ethylene homopolymer (A)having a melt flow index MI₂ of 5 to 1000 g/10 min and from 30 to 70% byweight with respect to the total weight of the composition of acopolymer of ethylene and of hexene (B) having a melt flow index MI₅ of0.01 to 2 g/10 min capable of being obtained by the process according toclaim 1 .
 11. Composition according to claim 10 , characterized in thatit exhibits an MI₅ of at least 0.07 and not exceeding 10 g/10 min and anHLMI/MI₅ ratio of greater than
 10. 12. Use of a composition according toclaim 10 for the manufacture of films or of pipes.
 13. Pipes obtained byextrusion of a composition according to claim 10 .